1. Field of the Invention
The present invention relates to a silencer provided on an exhaust pipe of a vehicle engine.
2. Description of the Related Art Japanese Unexamined Patent Application Publication No. 2008-138608 discloses an example of a silencer provided on an exhaust pipe of a vehicle engine. This silencer includes an inner cylinder 10 disposed at an intermediate position of the exhaust pipe 3; a housing 12 that accommodates the inner cylinder 10 and defines an expansion chamber 11 between the inner cylinder 10 and the housing 12; and a partition cylinder 17 accommodated in the housing 12, the partition cylinder 17 sectioning the expansion chamber 11 in the housing 12 into an inner expansion-chamber section 11a at an inner region in a radial direction and an outer expansion-chamber section 11b at an outer region in the radial direction, the outer expansion-chamber section 11b being filled with an acoustic material 23, the partition cylinder 17 having a group of small holes 22 through which the inner expansion-chamber section 11a and the outer expansion-chamber section 11b communicate with each other. The inner cylinder 10 has a group of small holes 21 through which the inside of the inner cylinder 10 communicates with the inner expansion-chamber section 11a. The exhaust sound that travels through the exhaust pipe 3 spreads into the inner expansion-chamber section 11a through the small holes 21 in the inner cylinder 10, and then spreads into the outer expansion-chamber section 11b through the small holes 22 in the partition cylinder 17, so that the sound pressure of the exhaust sound is attenuated in the outer expansion-chamber section 11b. 
In general, acoustic materials are highly effective in attenuating the sound pressure of a high-frequency component (500 Hz or more) of the exhaust sound. However, acoustic materials cannot effectively attenuate the sound pressure of a low-frequency component (100 Hz or less) of the exhaust sound. Therefore, the silencer disclosed in Japanese Unexamined Patent Application Publication No. 2008-138608 has a disadvantage that although the sound pressure of the high-frequency component can be effectively attenuated, the sound pressure of the low-frequency component cannot be sufficiently attenuated.
In general, the following measures are effective in attenuating the sound pressure of the low-frequency component of the exhaust sound:
(1) to reduce the diameter of the exhaust pipe, that is, to narrow the exhaust passage;
(2) to increase the length of the exhaust pipe, that is, to increase the length of the exhaust passage; and
(3) to increase the capacity of the expansion chamber.
However, measure (1) has a problem that a back pressure, which is a pressure that obstructs the discharge of the exhaust gas, will be increased and the engine output will be reduced. In addition, measure (2) has a problem that there may be a case where the length of the exhaust pipe cannot be increased owing to the restriction on the total vehicle length or the need to avoid interference with other components under the vehicle floor. Similarly, measure (3) has a problem that there may be a case where the capacity of the expansion chamber cannot be increased owing to the need to avoid interference with other components under the vehicle floor. Thus, each of measures (1) to (3) has a problem, and is difficult to carry out.